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Misted double glazing means the seal around the glass unit has failed and damp air has got in between the panes. It can’t be wiped away or permanently dried out, but you rarely need a new window. In most cases the sealed unit can be replaced on its own, in the existing frame, often in under an hour per window.
If you’ve spotted a milky haze that won’t clear, here’s what is happening, how to confirm it and what replacing the glass involves. For moisture on the room side of the glass, see our guide to condensation on windows.
What is a double glazed sealed unit?
A sealed unit, also called an insulated glass unit or IGU, is two panes of glass (three in triple glazing) held apart by a spacer bar around the edge and sealed to keep moisture out. The gap between the panes is filled with dry air or an insulating gas such as argon. That sealed, dry gap is what slows heat escaping through the glass.
Why does double glazing mist up between the panes?
Double glazing mists up when the edge seal of the unit breaks down and damp air gets into the gap. The spacer bar holds a desiccant that soaks up small amounts of moisture, but once the seal fails the desiccant is soon overwhelmed and condensation forms on the inner faces of the glass.
The most common causes of sealed unit failure are:
- Age. Seals weaken slowly, and many units start to fail somewhere between 10 and 25 years old.
- Sun and temperature changes. South and west facing windows heat up and cool down every day, which puts the seal under constant strain.
- Water sitting in the frame. If the drainage channels in the frame are blocked, the bottom edge of the unit can sit in water.
- Damage or poor fitting. A knock, a slammed sash or a unit that wasn’t packed properly when it was fitted can all stress the seal.
How can you tell if a sealed unit has failed?
A failed double glazed unit usually shows one or more of these signs:
- Mist or fog that you can’t wipe off from either side of the glass
- Haze that comes and goes as the temperature changes through the day
- Droplets or streaks of water trapped inside the glass
- White staining between the panes once the moisture dries
To check, wipe the glass inside the room, then outside. If the mist is still there, it is between the panes and the sealed unit has failed.
Can you replace just the glass in a double glazed window?
Yes. If the window frame, hinges and locks are in good condition, the failed sealed unit can be taken out and a new one fitted into the same frame. You keep your existing window, there’s no plastering or making good, and it costs a fraction of a full replacement window.
A whole new window is the better choice when:
- the frame is rotten, warped or cracked
- several units have failed at once and the frames are dated
- the window no longer opens, closes or locks properly
- you want better security or energy performance than the old frame can give
How does sealed unit replacement work?
- Measure. We measure the existing unit precisely and note its thickness, spacer bar colour and glass type, so the new one matches.
- Make. Your new sealed unit is made to measure. We’ll confirm the current lead time when we measure up.
- Remove. On fitting day the glazing beads are taken off and the old unit is lifted out.
- Fit. The new unit is set on packers so it sits square and drains properly, then the beads go back on.
- Check. We make sure the window opens, closes and locks smoothly, and tidy up before we leave.
Which glass options are available for a replacement sealed unit?
A replacement sealed unit is a chance to upgrade the glass as well as clear the view.
| Glass option | What it does | Where it suits |
| Energy efficient Low E glass | A thin coating reflects heat back into the room | Any window, especially living rooms and bedrooms |
| Toughened safety glass | Heat treated to be much stronger than ordinary glass, and breaks into small, blunt pieces | Doors, low level panes and other places where safety glass is required |
| Laminated glass | A clear interlayer holds the glass together if it breaks | Ground floor windows, extra security and noise reduction |
| Obscure glass | A pattern lets light in while blocking the view | Bathrooms, landings and front door side panels |
| Self cleaning glass | A coating helps rain wash dirt away | Conservatory roofs and hard to reach windows |
Is it worth replacing misted double glazing?
For most homes, yes. A failed sealed unit has lost its dry air or insulating gas, so the window lets more heat out, the glass looks permanently dirty and the misting only gets worse. Replacing the unit restores a clear view and the warmth of the glass without the cost of a new window.
Misted double glazing FAQs
Can misted double glazing be fixed without replacing the glass?
Some companies drill and dry out misted units. This can clear the view for a while, but it doesn’t restore the original edge seal or the insulating gas, so the unit won’t perform like new. For a lasting fix, we recommend replacing the sealed unit itself.
How long do double glazed sealed units last?
Most double glazed sealed units last somewhere between 10 and 25 years. The quality of the unit, how much sun the window gets and whether the frame drains properly all make a difference. Units in south facing windows and glazed doors often fail first.
Do I need a FENSA certificate to replace a sealed unit?
No. In England, replacing the glass alone in an existing frame isn’t notifiable building work, so no FENSA certificate is needed. Glass in doors and low level panes must still be safety glass to meet Building Regulations, and we’ll specify that for you when we measure.
Will a new sealed unit match my other windows?
In most cases, yes. We match the unit thickness, spacer bar colour and glass type, including obscure patterns where they’re still made. If an older pattern is no longer available, we’ll show you the closest options before anything is ordered.
Got a misted window?
Request a glass quote online or call 01932 344 415.
